systemd.journal-fields - Special journal fields
Entries in the journal (as written by
systemd-journald.service(8)) resemble a UNIX process environment
block in syntax but with field values that may include binary data, and with
non-unique field names permitted. Primarily, field values are formatted
UTF-8 text strings — binary encoding is used only where formatting as
UTF-8 text strings makes little sense. New fields may freely be defined by
applications, but a few fields have special meanings, which are listed
below. Typically, fields may only appear once per log entry, however there
are special exceptions: some fields may appear more than once per entry, in
which case this is explicitly mentioned below. Even though the logging
subsystem makes no restrictions on which fields to accept non-unique values
for, it is strongly recommended to avoid relying on this for the fields
listed below (except where listed otherwise, as mentioned) in order to avoid
unnecessary incompatibilities with other applications.
User fields are fields that are directly passed from clients and
stored in the journal.
The human-readable message string for this entry. This is
supposed to be the primary text shown to the user. It is usually not
translated (but might be in some cases), and is not supposed to be parsed for
metadata. In order to encode multiple lines in a single log entry, separate
them by newline characters (ASCII code 10), but encode them as a single
MESSAGE= field. Do not add multiple values of this field type to the
same entry (also see above), as consuming applications generally do not expect
this and are unlikely to show all values in that case.
A 128-bit message identifier ID for recognizing certain
message types, if this is desirable. This should contain a 128-bit ID
formatted as a lower-case hexadecimal string, without any separating dashes or
suchlike. This is recommended to be a UUID-compatible ID, but this is not
enforced, and formatted differently. Developers can generate a new ID for this
purpose with systemd-id128 new.
A priority value between 0 ("emerg") and 7
("debug") formatted as a decimal string. This field is compatible
with syslog's priority concept.
CODE_FILE=, CODE_LINE=, CODE_FUNC=
The code location generating this message, if known.
Contains the source filename, the line number and the function name.
The low-level Unix error number causing this entry, if
any. Contains the numeric value of errno(3)
formatted as a decimal
A randomized, unique 128-bit ID identifying each runtime
cycle of the unit. This is different from _SYSTEMD_INVOCATION_ID in
that it is only used for messages coming from systemd code (e.g. logs from the
system/user manager or from forked processes performing systemd-related
Syslog compatibility fields containing the facility
(formatted as decimal string), the identifier string (i.e. "tag"),
the client PID, and the timestamp as specified in the original datagram. (Note
that the tag is usually derived from glibc's
Note that the journal service does not validate the values of any
structured journal fields whose name is not prefixed with an underscore, and
this includes any syslog related fields such as these. Hence, applications
that supply a facility, PID, or log level are expected to do so properly
formatted, i.e. as numeric integers formatted as decimal strings.
The original contents of the syslog line as received in
the syslog datagram. This field is only included if the MESSAGE= field
was modified compared to the original payload or the timestamp could not be
located properly and is not included in SYSLOG_TIMESTAMP=. Message
truncation occurs when the message contains leading or trailing whitespace
(trailing and leading whitespace is stripped), or it contains an embedded
NUL byte (the NUL byte and anything after it is not included).
Thus, the original syslog line is either stored as SYSLOG_RAW= or it
can be recreated based on the stored priority and facility, timestamp,
identifier, and the message payload in MESSAGE=.
A documentation URL with further information about the
topic of the log message. Tools such as journalctl will include a
hyperlink to an URL specified this way in their output. Should be an
"http://", "https://", "file:/",
"man:" or "info:" URL.
The numeric thread ID (TID) the log message originates
The name of a unit. Used by the system and user managers
when logging about specific units.
When --unit=name or
--user-unit=name are used with journalctl(1), a
match pattern that includes "UNIT=name.service" or
"USER_UNIT=name.service" will be generated.
Fields prefixed with an underscore are trusted fields, i.e. fields
that are implicitly added by the journal and cannot be altered by client
_PID=, _UID=, _GID=
The process, user, and group ID of the process the
journal entry originates from formatted as a decimal string. Note that entries
obtained via "stdout" or "stderr" of forked processes will
contain credentials valid for a parent process (that initiated the connection
_COMM=, _EXE=, _CMDLINE=
The name, the executable path, and the command line of
the process the journal entry originates from.
The effective capabilities(7)
of the process the
journal entry originates from.
The session and login UID of the process the journal
entry originates from, as maintained by the kernel audit subsystem.
The control group path in the systemd hierarchy, the
systemd slice unit name, the systemd unit name, the unit name in the systemd
user manager (if any), the systemd session ID (if any), and the owner UID of
the systemd user unit or systemd session (if any) of the process the journal
entry originates from.
The SELinux security context (label) of the process the
journal entry originates from.
The earliest trusted timestamp of the message, if any is
known that is different from the reception time of the journal. This is the
time in microseconds since the epoch UTC, formatted as a decimal string.
The kernel boot ID for the boot the message was generated
in, formatted as a 128-bit hexadecimal string.
The machine ID of the originating host, as available in
The invocation ID for the runtime cycle of the unit the
message was generated in, as available to processes of the unit in
The name of the originating host.
How the entry was received by the journal service. Valid
for those read from the kernel audit subsystem
for internally generated messages
for those received via the local syslog socket with the
for those received via the native journal protocol
for those read from a service's standard output or error
for those read from the kernel
Only applies to "_TRANSPORT=stdout" records:
specifies a randomized 128bit ID assigned to the stream connection when it was
first created. This ID is useful to reconstruct individual log streams from
the log records: all log records carrying the same stream ID originate from
the same stream.
Only applies to "_TRANSPORT=stdout" records:
indicates that the log message in the standard output/error stream was not
terminated with a normal newline character ("\n", i.e. ASCII 10).
Specifically, when set this field is one of nul
(in case the line was
terminated by a NUL
(in case the maximum log
line length was reached, as configured with LineMax=
(if this was the last log record of a
stream and the stream ended without a final newline character), or
(if the process which generated the log output changed in
the middle of a line). Note that this record is not generated when a normal
newline character was used for marking the log line end.
If this file was written by a systemd-journald
instance managing a journal namespace that is not the default, this field
contains the namespace identifier. See systemd-journald.service(8)
details about journal namespaces.
A string field that specifies the runtime scope in which
the message was logged. If "initrd", the log message was processed
while the system was running inside the initrd. If "system", the log
message was generated after the system switched execution to the host root
Kernel fields are fields that are used by messages originating in
the kernel and stored in the journal.
The kernel device name. If the entry is associated to a
block device, contains the major and minor numbers of the device node,
separated by ":" and prefixed by "b". Similarly for
character devices, but prefixed by "c". For network devices, this is
the interface index prefixed by "n". For all other devices, this is
the subsystem name prefixed by "+", followed by ":",
followed by the kernel device name.
The kernel subsystem name.
The kernel device name as it shows up in the device tree
The device node path of this device in /dev/.
Additional symlink names pointing to the device node in
/dev/. This field is frequently set more than once per entry.
Fields in this section are used by programs to specify that they
are logging on behalf of another program or unit.
Fields used by the systemd-coredump coredump kernel
Used to annotate messages containing coredumps from
system and session units. See coredumpctl(1)
Privileged programs (currently UID 0) may attach
OBJECT_PID= to a message. This will instruct systemd-journald
to attach additional fields on behalf of the caller:
PID of the program that this message pertains to.
OBJECT_UID=, OBJECT_GID=, OBJECT_COMM=,
OBJECT_EXE=, OBJECT_CMDLINE=, OBJECT_AUDIT_SESSION=,
These are additional fields added automatically by
systemd-journald. Their meaning is the same as _UID=,
_GID=, _COMM=, _EXE=, _CMDLINE=,
_AUDIT_SESSION=, _AUDIT_LOGINUID=, _SYSTEMD_CGROUP=,
_SYSTEMD_SESSION=, _SYSTEMD_UNIT=, _SYSTEMD_USER_UNIT=,
and _SYSTEMD_OWNER_UID= as described above, except that the process
identified by PID is described, instead of the process which logged the
During serialization into external formats, such as the Journal
Export Format or the Journal JSON Format, the addresses of
journal entries are serialized into fields prefixed with double underscores.
Note that these are not proper fields when stored in the journal but for
addressing metadata of entries. They cannot be written as part of structured
log entries via calls such as sd_journal_send(3). They may also not
be used as matches for sd_journal_add_match(3).
The cursor for the entry. A cursor is an opaque text
string that uniquely describes the position of an entry in the journal and is
portable across machines, platforms and journal files.
The wallclock time (CLOCK_REALTIME) at the point
in time the entry was received by the journal, in microseconds since the epoch
UTC, formatted as a decimal string. This has different properties from
"_SOURCE_REALTIME_TIMESTAMP=", as it is usually a bit later but more
likely to be monotonic.
The monotonic time (CLOCK_MONOTONIC) at the point
in time the entry was received by the journal in microseconds, formatted as a
decimal string. To be useful as an address for the entry, this should be
combined with the boot ID in "_BOOT_ID=".
- Journal Export Format
- Journal JSON Format